43 2 PHYSICAL GEOGRAPHY. 



placed near to the centre or at points midway between the centre and 

 the edge, and half-way between the horn-like projections. 



In outline, Biddulphia passes into a genus known as Triceratium, 

 which, as its name indicates, is provided with three projecting horns or 

 corners. In fact, what may be called the normal form of its valve is 

 triangular, having three horn-like projecting portions like the two in 

 Biddulphia. But although the commonest outline is triangular, we find 

 certain species varying to such an extent as to have examples with four, 

 five, and even six sides, — in this respect resembling Aulacodiscus, whose 

 number of projections, or " feet," vary in the manner described. And 

 there is a genus called Amphitetras, which apparently only differs from 

 Triceratium in the fact that its normal form is with four corners. In 

 truth, we find occasionally specimens of Biddulphia with three corners, 

 Triceratium with four, five, six, and even nine corners, and Amphitetras 

 with five corners, so it becomes extremely difficult to draw lines of dis- 

 tinction between these three genera. Besides this, we find that among 

 themselves the species of Triceratium differ in minor characters ; some 

 have the sides convex, becoming more and more so, until at last we have a 

 perfectly circular outline still retaining the three projecting horns. Then 

 we find them with sides straight, then more and more concave, until the 

 valve appears to be but three arms united by a very small body. Some 

 have undulate sides. The front view is as various as the side view. In 

 some the processes are nearly level with the surface of the valve, while 

 others have them considerably elevated, and attenuated into spines. So, 

 again, in this beautiful genus, the sculpturing of the valve is very various. 

 We have coarse hexagonal reticulations, with or without finer ones within 

 them, fine hexagonal markings, circular, dot-like, radiant or curved depres- 

 sions, in some cases of such delicacy that high-power glasses are required 

 for their elimination. Then, again, we have large, heavy bars of silica 

 projecting across the valve in different directions, merely cutting off the 

 corners, or dividing the central portion in various ways. In short, Tricera- 

 tium is one of the most variable as it is one of the most beautiful genera 

 of the diatomacenc. It is found living in the ocean, growing in chains 

 attached to algae and shells, after the manner of Biddulphia and Melosira. 

 Some of the most beautiful species have been as yet found only in the 

 fossil condition in certain so-called "infusorial" earths. 



